Telescopic type hydraulic actuators are known that comprise a cylinder defining a spherical cavity and a rod that extends through one end of the cavity, being secured to a piston that is mounted to slide axially inside the cavity so as to define two hydraulic chambers therein. During a movement of the rod, it is known to organize throttling on the outlet from the chamber from which fluid is expelled in order to perform hydraulic damping. Such throttling generates resistance that is proportional to the square of the travel speed of the rod.
Electrohydraulic actuators are also known that have an electric pump arranged to move fluid from one chamber to the other of the actuator. It is also possible to organize damping by throttling the fluid that is expelled from one of the chambers during movement of the rod. Nevertheless, such an actuator is not suitable for emergency operation in which the rod is driven by the load to which it is coupled, unless the pump is reversible and is capable of being released.
Mechanical actuators are also known that comprise a screw extending inside the rod for co-operating with a nut secured to the rod, the screw being driven in rotation by a motor, which on being activated causes the rod to move axially. In such actuators, damping may be provided by organizing resistance, e.g. of the friction type or of the electromagnetic type, against movement of the rod. Nevertheless, the first type of resistance is essentially independent of the travel speed of the rod, while the second type of resistance is rather difficult to organize and in general is a function of the travel speed of the rod, and that might be found to be insufficient in certain applications. In addition, such damping leads to numerous problems in service, such as wear or lack of reliability.
Finally, document FR 2 946 618 is known, which discloses a hydraulically-damped electromechanical actuator having a cylinder in which a rod is mounted to slide, the actuator having a screw that extends inside the rod to co-operate with a nut secured to the rod, the screw being driven in rotation by a motor. The rod is associated with a piston that slides inside a cavity of the cylinder in order to define two chambers therein, the chambers being filled with hydraulic fluid and being put into communication by transfer means fitted with at least one regulator member adapted to throttling the fluid expelled from one of the chambers, for at least one travel direction of the rod.
In that type of actuator, both chambers are permanently filled with hydraulic fluid, and in general they need to be connected to an accumulator in order to deliver or receive differential flow rates between the two chambers. Fluid transfer, and thus hydraulic damping, then accompanies each of the movements of the rod, whereas in normal operation such damping is not always necessary.